CHAPTER 5ION CHANNEL

Molecules are diffused into the cell

1. Lipid soluble molecules

2.Through channels

Plasma membrane

FIGURE 3-15 DIFFUSION ACROSS THE PLASMA MEMBRANE

EXTRACELLULAR FLUID

CYTOPLASM

Lipid-soluble moleculesdiffuse through theplasma membrane

Channelprotein

Small water-solublemolecules and ionsdiffuse throughmembrane channels

Large molecules that cannotdiffuse through lipids cannotcross the plasma membraneunless they are transportedby a carrier mechanism

Molecules are actively transported into the cell

What means ‘actively’?

FIGURE 3-19 THE SODIUM-POTASSIUM EXCHANGE PUMP

EXTRACELLULARFLUID

Sodium-potassiumexchange

pump

CYTOPLASM

Endocytosis: getting molecules inside

Exocytosis: spitting molecules outside

vs.

THE PERMEABILITY OF THE CELL MEMBRANE

Cell membrane

Permeable?

HOW TO DETECT THE CURRENT?

CHARACTERISTICS OF THE CURRENT

CURRENT-VOLTAGE

Ohmic channel Rectifying channel

Characters of channels

CHANNEL STRUCTURE & FUNCTION

THE NOBEL PRIZE IN CHEMISTRY 2003

Roderick MacKinnon Peter Agre

The Nobel Prize in Chemistry 2003 was awarded "for discoveries concerning channels in cell membranes" jointly with one half to Peter Agre "for the discovery of water channels" and with one half to Roderick MacKinnon "for structural and mechanistic studies of ion channels".

THE NOBEL PRIZE IN CHEMISTRY 2003

MACKINNON PAPERS

POTASSIUM CHANNELS AND THE ATOMIC BASISOF SELECTIVE ION CONDUCTION

Nobel Lecture, December 8, 2003

by

Roderick MacKinnon

Howard Hughes Medical Institute, Laboratory of Molecular Neurobiologyand Biophysics, Rockefeller University, 1230 York Avenue, New York, NY 10021, USA.

INTRODUCTION

All living cells are surrounded by a thin, approximately 40 Å thick lipid bilay-er called the cell membrane. The cell membrane holds the contents of a cellin one place so that the chemistry of life can occur, but it is a barrier to themovement of certain essential ingredients including the ions Na+, K+, Ca2+

and Cl-. The barrier to ion flow across the membrane – known as the dielec-tric barrier – can be understood at an intuitive level: the cell membrane inte-rior is an oily substance and ions are more stable in water than in oil. The en-ergetic preference of an ion for water arises from the electric field around theion and its interaction with neighboring molecules. Water is an electricallypolarizable substance, which means that its molecules rearrange in an ion’selectric field, pointing negative oxygen atoms in the direction of cations andpositive hydrogen atoms toward anions. These electrically stabilizing interac-tions are much weaker in a less polarizable substance such as oil. Thus, an ionwill tend to stay in the water on either side of a cell membrane rather than en-ter and cross the membrane. And yet numerous cellular processes, rangingfrom electrolyte transport across epithelia to electrical signal production inneurons, depend on the flow of ions across the membrane. To mediate theflow, specific protein catalysts known as ion channels exist in the cell mem-brane. Ion channels exhibit the following three essential properties: (1) theyconduct ions rapidly, (2) many ion channels are highly selective, meaning onlycertain ion species flow while others are excluded, (3) their function is regu-lated by processes known as gating, that is, ion conduction is turned on andoff in response to specific environmental stimuli. Figure 1 summarizes theseproperties (figure 1).

The modern history of ion channels began in 1952 when Hodgkin andHuxley published their seminal papers on the theory of the action potentialin the squid giant axon (Hodgkin and Huxley, 1952a; Hodgkin and Huxley,1952b; Hodgkin and Huxley, 1952c; Hodgkin and Huxley, 1952d). A funda-mental element of their theory was that the axon membrane undergoeschanges in its permeability to Na+ and K+ ions. The Hodgkin-Huxley theory

214

The Structure of the PotassiumChannel: Molecular Basis of K!

Conduction and SelectivityDeclan A. Doyle, Joao Morais Cabral, Richard A. Pfuetzner,

Anling Kuo, Jacqueline M. Gulbis, Steven L. Cohen,Brian T. Chait, Roderick MacKinnon*

The potassium channel from Streptomyces lividans is an integral membrane protein withsequence similarity to all known K! channels, particularly in the pore region. X-rayanalysis with data to 3.2 angstroms reveals that four identical subunits create an invertedteepee, or cone, cradling the selectivity filter of the pore in its outer end. The narrowselectivity filter is only 12 angstroms long, whereas the remainder of the pore is widerand lined with hydrophobic amino acids. A large water-filled cavity and helix dipoles arepositioned so as to overcome electrostatic destabilization of an ion in the pore at thecenter of the bilayer. Main chain carbonyl oxygen atoms from the K! channel signaturesequence line the selectivity filter, which is held open by structural constraints to co-ordinate K! ions but not smaller Na! ions. The selectivity filter contains two K! ions about7.5 angstroms apart. This configuration promotes ion conduction by exploiting electro-static repulsive forces to overcome attractive forces between K! ions and the selectivityfilter. The architecture of the pore establishes the physical principles underlying selectiveK! conduction.

Potassium ions diffuse rapidly across cellmembranes through proteins called K!

channels. This movement underlies manyfundamental biological processes, includ-ing electrical signaling in the nervous sys-tem. Potassium channels use diversemechanisms of gating (the processes bywhich the pore opens and closes), but theyall exhibit very similar ion permeabilitycharacteristics (1). All K! channels showa selectivity sequence of K! " Rb! #Cs!, whereas permeability for the smallestalkali metal ions Na! and Li! is immea-surably low. Potassium is at least 10,000times more permeant than Na!, a featurethat is essential to the function of K!

channels. Potassium channels also share aconstellation of permeability characteris-tics that is indicative of a multi-ionconduction mechanism: The flux of ionsin one direction shows high-order cou-pling to flux in the opposite direction, andionic mixtures result in anomalous con-duction behavior (2). Because of these

properties, K! channels are classifiedas “long pore channels,” invoking thenotion that multiple ions queue inside along, narrow pore in single file. Inaddition, the pores of all K! channelscan be blocked by tetraethylammonium(TEA) ions (3).

Molecular cloning and mutagenesis ex-periments have reinforced the conclusionthat all K! channels have essentially thesame pore constitution. Without exception,all contain a critical amino acid sequence,the K! channel signature sequence. Muta-tion of these amino acids disrupts the chan-nel’s ability to discriminate between K!

and Na! ions (4).Two aspects of ion conduction by K!

channels have tantalized biophysicists forthe past quarter century. First, what is thechemical basis of the impressive fidelitywith which the channel distinguishes be-tween K! and Na! ions, which are feature-less spheres of Pauling radius 1.33 Å and0.95 Å, respectively? Second, how can K!

channels be so highly selective and at thesame time, apparently paradoxically, exhib-it a throughput rate approaching the diffu-sion limit? The 104 margin by which K! isselected over Na! implies strong energeticinteractions between K! ions and the pore.And yet strong energetic interactions seemincongruent with throughput rates up to108 ions per second. How can these twoessential features of the K! channel pore bereconciled?

Potassium Channel Architecture

The amino acid sequence of the K! chan-nel from Streptomyces lividans (KcsA K!

channel) (5) is similar to that of other K!

channels, including vertebrate and inverte-brate voltage-dependent K! channels, ver-tebrate inward rectifier and Ca2!-activatedK! channels, K! channels from plants andbacteria, and cyclic nucleotide-gated cationchannels (Fig. 1). On the basis of hydro-phobicity analysis, there are two closelyrelated varieties of K! channels, those con-taining two membrane-spanning segmentsper subunit and those containing six. In allcases, the functional K! channel protein isa tetramer (6), typically of four identicalsubunits (7). Subunits of the two mem-brane-spanning variety appear to be short-ened versions of their larger counterparts, asif they simply lack the first four membrane-spanning segments. Although the KcsA K!

channel is a two membrane-spanning K!

channel, its amino acid sequence is actuallycloser to those of eukaryotic six membrane-spanning K! channels. In particular, itssequence in the pore region, located be-tween the membrane-spanning stretchesand containing the K! channel signaturesequence, is nearly identical to that foundin the Drosophila (Shaker) and vertebratevoltage-gated K! channels (Fig. 1). In anaccompanying paper, through a study of theKcsA K! channel interaction with eukary-otic K! channel toxins, we confirm thatthe KcsA pore structure is indeed very sim-ilar to that of eukaryotic K! channels andthat its structure is maintained when it isremoved from the membrane with deter-gent (8).

We have determined the KcsA K!

channel structure from residue position 23to 119 by x-ray crystallography (Table 1).The cytoplasmic carboxyl terminus (resi-dues 126 to 158) was removed in the prep-aration and the remaining residues weredisordered. The KcsA K! channel crystalsare radiation-sensitive and the diffractionpattern is anisotropic, with reflections ob-served along the best and worst directionsat 2.5 Å and 3.5 Å Bragg spacings, respec-tively. By data selection, anisotropy correc-tion, introduction of heavy atom sites bysite-directed mutagenesis, averaging, andsolvent flattening, an interpretable electrondensity map was calculated (Fig. 2, Athrough C). This map was without mainchain breaks and showed strong side chaindensity (Fig. 2C). The model was refinedwith data to 3.2 Å (the data set was 93 %complete to 3.2 Å with 67% completenessbetween 3.3 Å and 3.2 Å), maintaininghighly restrained stereochemistry and keep-ing tight noncrystallographic symmetry re-straints. The refinement procedure was

D. A. Doyle, R. A. Pfuetzner, A. Kuo, and R. MacKinnonare in the Laboratory of Molecular Neurobiology and Bio-physics and the Howard Hughes Medical Institute, Rock-efeller University, 1230 York Avenue, New York, NY10021, USA. J. M. Cabral and J. M. Gulbis are in theLaboratory of Molecular Neurobiology and Biophysics,Rockefeller University, 1230 York Avenue, New York, NY10021, USA. S. L. Cohen and B. T. Chait are in theLaboratory of Mass Spectrometry and Gaseous IonChemistry, Rockefeller University, 1230 York Avenue,New York, NY 10021, USA.

*To whom correspondence should be addressed. E-mail:mackinn@rockvax.rockefeller.edu

RESEARCH ARTICLES

www.sciencemag.org ! SCIENCE ! VOL. 280 ! 3 APRIL 1998 69

on

Augu

st 8

, 201

1w

ww

.sci

ence

mag

.org

Dow

nloa

ded

from

THE NOBEL PRIZE IN CHEMISTRY 2003

Passage of water molecules through the aquaporin AQP1. Because of the positive charge at the center of the channel, positively charged ions such as H3O+, are deflected. This prevents proton leakage through the channel.

THE NOBEL PRIZE IN CHEMISTRY 2003

The ion channel permits passage of potassium ions but not sodium ions. The oxygen atoms of the ion filter form an environment very similar to the water environment outside the filter. The cell may also control opening and closing of the channel.

HOW WORKS?

THREE PHYSICAL MODEL OF CHANNEL

HOW WORKS?

?

SEVERAL TYPES OF STIMULI CONTROL THE OPENING AND CLOSING

HOW WORKS?

Inactivation

VOLTAGE-GATED CHANNELS: TWO MECHANISMS

EXOGENOUS LIGANDS CAN BIAS AN ION CHANNEL TOWARD AN OPEN OR CLOSED STATE.

STRUCTURE OF CHANNEL

THE SECONDARY STRUCTURE OF MEMBRANE-SPANNING PROTEINS

Type 1 & II ?

THREE SUPERFAMILIES

P REGION

¡ The more external (i.e., more extracellular) portion of the pore is formed by the "P-loops" (the region between S5 and S6) of the four domains. This region is the most narrow part of the pore and is responsible for its ion selectivity.

FOUR RELATED FAMILIES OF ION CHANNELS WITH P-REGIONS

FOUR RELATED FAMILIES OF ION CHANNELS WITH P-REGIONS

(Sandoz & Levitz, 2013)

FOUR RELATED FAMILIES OF ION CHANNELS WITH P-REGIONS

Honor, 2007

GENE EXPRESSION IN DEVELOPMENT

DIFFERENT CHANNELS IN DIFFERENT REGIONS

X-RAY CRYSTAL STRUCTURE

GATING OF BACTERIAL POTASSIUM CHANNELS

CIC FAMILY OF CHLORIDE CHANNELS AND TRANSPORTERS

Looks different?

COMPARISON OF GENERAL ARCHITECTURE

Parallel Anti-parallel

SPECIFICITY

HOW TO GET THE SPECIFICITY?

https://www.youtube.com/watch?v=4zms9bXM2FA

PUMPS VS. ION CHANNELS

NEXT…

Signal processing…

CHAPTER 06.

MEMBRANE POTENTIAL AND THE PASSIVE ELECTRICAL PROPERTIES OF THE NEURON

NeuronalconditionMembrane

Potential, Vm

Vm = Vin -Vout

THREE METHODS TO RECORD NEURON

PATCH CLAMP RECORDING

EXTRACELLULAR RECORDING

INTRA VS. EXTRACELLULAR RECORDING

Resting potential: -60 ~ - 70 mV

Imagine…

Depolarization

Hyperpolarization

How do they send information through their long neurites?

How do they send information through their long neurites?

How do they send information through their long neurites?

Action potential(Cole & Curtis, 1939)

(Cole & Curtis, 1939)

Action potential

THE ACTION POTENTIAL IS A RAPID CHANGE IN MEMBRANE POTENTIAL

1. Depolarizationphase

2. Repolarizationphase

3. Hyperpolarization phase

Resting potential

Threshold potential

VOLTAGE-GATED CHANNELS

How voltage-gated channels work

At the resting potential, voltage-gated Na+ channels are closed.

Conformational changes openvoltage-gated channels whenthe membrane is depolarized.

Two important types:1.) Na+ voltage gated channels2.) K+ voltage gated channels

Resting Potential - Both voltage gated Na+ and K+ channels are closed.

Simplified model of AP

Initial Depolarization - Some Na+ channels open. If enough Na+ channels open, then the threshold is surpassed and an action potential is initiated.

Na+ channels open quickly. K+ channels are still closed.

PNa+ > PK+

Na+ channels self-inactivate, K+ channels are open.

PK+ >> PNa+

Emembrane ≈ E K+

PK+ > PK+ at resting state

Resting Potential - Both Na+ and K+ channels are closed.

https://www.youtube.com/watch?v=OZG8M_ldA1M

Action potential

https://www.youtube.com/watch?v=ifD1YG07fB8

NEXT…

How to travel the long, long way through axon?

READING PAPER

Nature © Macmillan Publishers Ltd 1997

The capsaicin receptor: a

heat-activated ion channel

in the pain pathway

Michael J. Caterina*,MarkA.Schumacher†k,MakotoTominaga*k, TobiasA.Rosen*, JonD. Levine‡ &David Julius*

Departments of * Cellular and Molecular Pharmacology, † Anesthesia, and ‡ Medicine, University of California, San Francisco, California 94143-0450, USAk These authors contributed equally to this study.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Capsaicin, the main pungent ingredient in ‘hot’ chilli peppers, elicits a sensation of burning pain by selectively

activating sensoryneuronsthat convey information about noxiousstimuli to thecentral nervoussystem.Wehaveused

anexpressioncloningstrategy basedoncalcium influx to isolate a functional cDNAencodingacapsaicin receptor from

sensory neurons. This receptor is a non-selective cation channel that is structurally related to members of the TRP

family of ion channels. The cloned capsaicin receptor is also activated by increases in temperature in the noxious

range, suggesting that it functions as a transducer of painful thermal stimuli in vivo.

Pain is initiated when the peripheral terminals of a subgroup ofsensory neurons are activated by noxious chemical, mechanical orthermal stimuli. These neurons, called nociceptors, transmit infor-mation regarding tissue damage to pain-processing centres in thespinal cord and brain1. Nociceptors are characterized, in part, bytheir sensitivity to capsaicin, a natural product of capsicum peppersthat is the active ingredient of many ‘hot’ and spicy foods. Inmammals, exposure of nociceptor terminals to capsaicin leadsinitially to excitation of the neuron and the consequent perceptionof pain and local release of inflammatory mediators. With pro-longed exposure, nociceptor terminals become insensitive to cap-saicin, as well as to other noxious stimuli2. This latter phenomenonof nociceptor desensitization underlies the seemingly paradoxicaluse of capsaicin as an analgesic agent in the treatment of painfuldisorders ranging from viral and diabetic neuropathies to rheuma-toid arthritis3,4. Some of this decreased sensitivity to noxious stimulimay result from reversible changes in the nociceptor, but the long-term loss of responsiveness can be explained by death of thenociceptor or destruction of its peripheral terminals followingexposure to capsaicin2,5.

The cellular specificity of capsaicin action and its ability to evokethe sensation of burning pain have led to speculation that the targetof capsaicin action plays an important physiological role in thedetection of painful stimuli. Indeed, capsaicin may elicit theperception of pain by mimicking the actions of a physiologicalstimulus or an endogenous ligand produced during tissue injury6.Although the excitatory and neurotoxic properties of capsaicinhave been used extensively to define and study nociceptiveneurons, its precise mechanism of action has remained elusive.Electrophysiological7,8 and biochemical9 studies have shown thatcapsaicin excites nociceptors by increasing the permeability of theplasma membrane to cations, but the molecular mechanism under-lying this phenomenon is unclear. Proposed models range from thedirect perturbation of membrane lipids by hydrophobic capsaicinmolecules10 to the activation of a specific receptor on or withinsensory neurons6. Because capsaicin derivatives show structure–function relationships and evoke responses in a dose-dependentmanner11,12, the existence of a receptor site represents the most likelymechanism. This model has been strengthened by the developmentof capsazepine, a competitive capsaicin antagonist13, and by thediscovery of resiniferatoxin, an extremely potent capsaicin analoguefrom Euphorbia plants that mimics the cellular actions ofcapsaicin14,15. The potency of resiniferatoxin at nanomolar quantities

has led to its use as a high-affinity radioligand to visualize saturable,capsaicin- and capsazepine-sensitive binding sites on nociceptors16.

A more detailed understanding of the molecular nature ofcapsaicin action and its relationship to endogenous pain signallingmechanisms might be obtained through the cloning of a geneencoding a capsaicin receptor. To achieve this we used a functionalscreening assay to isolate a cDNA clone that reconstitutes capsaicinresponsiveness in non-neuronal cells. The deduced amino-acidsequence of this clone demonstrates that the capsaicin receptor isan integral membrane protein with homology to a family of putativestore-operated calcium channels. The cloned receptor seems to be

articles

816 NATURE | VOL 389 | 23 OCTOBER 1997

Figure 1 Expression cloning of a capsaicin receptor using calcium imaging.

HEK293 cells transiently transfected with pools of clones from a rodent dorsal root

ganglion (DRG) cDNA library were subjected to microscopic fluorescent calcium

imaging before (left) and during (right) treatment with 3 mM capsaicin. Cells

transfected with vector alone (pCDNA3; top) exhibited no response to capsaicin.

Between 1% and 5% of cells transfected with pool 11 exhibited marked increases

in cytoplasmiccalcium (middle, arrowheads). This poolwas iteratively subdivided

and reassayed until a single positive clone (VR1) was isolated (bottom). Elevated

relative calcium concentrations are indicated by an increased ratio of Fura-2

emission at 340 versus 380nm wavelength excitation (see colour bar).

READING PAPER

Development/Plasticity/Repair

Nigrostriatal Dopaminergic Neurodegeneration in theWeaver Mouse Is Mediated via Neuroinflammation andAlleviated by Minocycline Administration

Jun Peng,1 Lin Xie,1 Fang Feng Stevenson,1 Simon Melov,1 Donato A. Di Monte,2 and Julie K. Andersen1

1Buck Institute for Age Research, Novato, California 94945, and 2Parkinson’s Institute, Sunnyvale, California 94089

The murine mutant weaver (gene symbol, wv) mouse, which carries a mutation in the gene encoding the G-protein inwardly rectifyingpotassium channel Girk2, exhibits a diverse range of defects as a result of postnatal cell death in several different brain neuron subtypes.Loss of dopaminergic nigrostriatal neurons in the weaver, unlike cerebellar granule neuronal loss, is via a noncaspase-mediated mech-anism. Here, we present data demonstrating that degeneration of midbrain dopaminergic neurons in weaver is mediated via neuroin-flammation. Furthermore, in vivo administration of the anti-inflammatory agent minocycline attenuates nigrostriatal dopaminergicneurodegeneration. This has novel implications for the use of the weaver mouse as a model for Parkinson’s disease, which has beenassociated with increased neuroinflammation.

Key words: weaver; dopaminergic midbrain neurons; caspase-independent cell death; microglial activation; minocycline; Parkinson

IntroductionWeaver (wv) is a naturally occurring murine mutation in theGirk2 gene, which encodes a G-protein-activated inwardly recti-fying potassium ion channel (Patil et al., 1995). Mice carrying twocopies of the mutant gene display ataxia, hyperactivity, andtremor (Caviness and Rakic, 1978). These neurological defectsare associated with neuronal degeneration during postnatal de-velopment in the ventral midbrain among dopaminergic neuronsof the substantia nigra (SN) and retrorubral nucleus (Schmidt etal., 1982; Roffler-Tarlov and Graybiel, 1984; Triarhou et al., 1988;Graybiel et al., 1990; Roffler-Tarlov et al., 1996), within granulecells (Rezai and Yoon, 1972; Rakic and Sidman, 1973a,b) andPurkinje cells (Blatt and Eisenman, 1985; Herrup and Trenkner,1987; Smeyne and Goldowitz, 1990) of the cerebellum and thedeep cerebellar nuclei (Maricich et al., 1997). The Girk2wv defectappears to initiate different types of cell death based on the mor-phological appearance of dying neurons in these different brainregions. Affected cerebellar granule neurons display the morpho-logical characteristics of classic apoptosis, including chromatincondensation and aggregation to the nuclear margin, cytoplas-mic shrinkage, and membrane blebbing (Savitz and Rosenbaum,1998). In addition, discreet chromatin cleavage as indicated byDNA end-labeling is also observed in dying granule neurons(Smeyne and Goldowitz, 1989; Harrison and Roffler-Tarlov,1998). In contrast, dying midbrain dopaminergic neurons can-

not be detected by assays for end-labeled cells and do not displayclassic morphological features of apoptosis (Oo et al., 1996;Migheli et al., 1997, 1999). Instead, cell death is characterized bylack of chromatin clumping, maintenance of neuronal shape, andunshrunken cytoplasm in the retrorubral nucleus (A8) and thesubstantia nigra pars compacta (SNpc) (A9) regions, whereas theadjacent A10, which contains dopaminergic cells in the medialportion of the ventral midbrain, is mostly spared (Roffler-Tarlovand Graybiel, 1984, 1986). The selective vulnerability in the ven-tral midbrain is explained by the fact that the dopaminergic neu-rons of the A8 and A9 express Girk2, whereas little or no Girk2 isfound within A10 neurons (Schein et al., 1998). However, themolecular mechanisms involved in Girk2wv-mediated dopami-nergic cell death are not known.

To verify that dopaminergic cell death in the midbrains ofweaver mice does not occur via apoptosis, we expressed the gen-eral caspase inhibitor baculoviral p35 and examined cell loss inthe midbrain. Expression of p35 did not attenuate death of do-paminergic neurons in the weaver ventral midbrain (supplemen-tal data, available at www.jneurosci.org as supplemental mate-rial). To better understand the mechanisms underlying selectivedopaminergic cell death in this brain region, we performed mi-croarray analyses to assess gene expression in the weaver SN dur-ing the period of selective dopaminergic neurodegeneration. Wediscovered, among alterations in expression of other genes, anelevation in expression levels of the mRNA for the microglial-associated inflammatory gene !2-microglobulin and several ma-jor histocompatibility complex (MHC) class I proteins. Thesechanges were subsequently found to be associated with increasedexpression of the corresponding proteins within nigral dopami-nergic neurons in the weaver midbrain along with increased levelsof microglial activation. Minocycline, an anti-inflammatoryagent and inhibitor of microglial activation, was found to atten-

Received April 30, 2006; revised Sept. 21, 2006; accepted Sept. 22, 2006.This work was supported by grants from the National Institutes of Health (J.K.A., S.M.). We thank Dr. Susan

Roffler-Tarlov for advice and discussion on morphological issues related to the p35 transgenic expression studies inthe weaver mice.

Correspondence should be addressed to Julie K. Andersen, Buck Institute for Age Research, 8001 RedwoodBoulevard, Novato, CA 94945. Email: jandersen@buckinstitute.org.

DOI:10.1523/JNEUROSCI.3447-06.2006Copyright © 2006 Society for Neuroscience 0270-6474/06/2611644-08$15.00/0

11644 • The Journal of Neuroscience, November 8, 2006 • 26(45):11644 –11651

ACTION POTENTIALS AND CONDUCTION

Neuron

F8-2

• Axons carry information from the cell body to the axon terminals.

• Axon terminals communicate with their target cells at synapses.

T3-5

• Difference in ion concentration between compartments gives rise to the resting membrane potential (RMP). Membrane permeability to these ions also influences the RMP.

• Transient changes from the RMP produce electrical signals which transmit information in nerve cells.

Changes in the Membrane Potential Produce Electric Signals in Nerve Cells

Terminology Associated with Changes in Membrane Potential

F8-7, F8-8

• Depolarization- a decrease in the potential difference between the inside and outside of the cell.

•Hyperpolarization- an increase in the potential difference between the inside and outside of the cell.

• Repolarization- returning to the RMP from either direction.

•Overshoot- when the inside of the cell becomes +ve due to the reversal of the membrane potential polarity.

• In the nervous system, different channel types are responsible for transmitting electrical signals over long and short distances:

•A) Graded potentials travel over short distances and are activated by the opening of mechanically or chemically gated channels.

•B) Action potentials travel over long distances and they are generated by the opening of voltage-gated channels.

Gated Channels Are Involved in Neuronal Signalling

Graded Potentials

F8-9

•Graded potentials are depolarizations or hyperpolarizations whose strength is proportional to the strength of the triggering event.

•Graded potentials lose their strength as they move through the cell due to the leakage of charge across the membrane (eg. leaky water hose).

Frequency of Action Potential Firing is Proportional to the Size of the Graded

Potential

The amount of neurotransmitter released from the axon terminal is proportional to the frequency of action potentials.

F8-18

•A graded potential depolarization is called excitatory postsynaptic potential (EPSP). A graded potential hyperpolarization is called an inhibitory postsynaptic potentials(IPSP).

•They occur in the cell body and dendrites of the neuron.

•The wave of depolarization or hyperpolarization which moves through the cell with a graded potential is known as local current flow.

Question: EPSP or IPSP?

Question: See through the AP!!!

•Graded potentials travel through the neuron until they reach the trigger zone. If they depolarize the membrane above threshold voltage (about -55 mV in mammals), an action potential is triggered and it travels down the axon.

F8-10

Graded Potentials Above Threshold Voltage Trigger Action Potentials

What happens?

Spatial Summation

• A neuron may receive greater than 10, 000 inputs from presynaptic neurons.

• The initiation of an action potential from several simultaneous subthreshold graded potentials, originating from different locations, is known as spatial summation.

F8-12

Temporal Summation

• When summation occurs from graded potentials overlapping in time, it is called temporal summation.

• Summation of graded potentials demonstrates a key property of neurons: postsynaptic integration.

F8-13

Action Potential (AP)

• They are initiated in an all-or-none manner when the summed graded potential exceed threshold voltage.

•They remain the same size as they travel along the axon over long distances.

• They are identical to one another.

• Occurs upon alteration of the permeability of Na+ and K+ ions through voltage-gated channels.

F8-14

“0 or 1”

Graded Potential vs Action Potential

Timecourse of the Action Potential

Na+ Channels Have Two Gates

F8-15

• The movement of the inactivation gate is coupled to the movement of the activation gate, but its response time is slower.

• When the activation gate is open, the signal passes along the channel protein to the inactivation gate.

•Na+/K+-ATPase Has No Direct Role to Play in the Action Potential.

• Absolutely refractory period- a second AP will not occur until the first is over. The gates on the Na+ channel have not reset.

•Relatively refractory period- a large suprathreshold graded potential can start a second AP by activating Na+ channels which have been reset.

Refractory Periods Limit the Frequency of APs

F8-17

Action Potential Conduction

• Movement of the AP along the axon at high speed is called conduction.

• A wave of action potentials travel down the axon.

• Each section of the axon is experiencing a different phase of the AP (see figure).

F8-19

If you stimulated here…

If you stimulated here…

???

If you stimulated here…

•Absolute refractory periods prevent back propagation of APs into the cell body.

•Refractory periods limit the rate at which signals can be transmitted down a neuron. Limit is around 100 impulses/s.

• Graded potential triggers AP. Opens voltage-gated Na+ channels.

F8-20a

•The Na+ spreads in all directions attracted by the -ve ions in adjacent regions (3,4). Opens Na+ channels and initiates AP in the adjacent region along the axon (4), but not in the cell body where there are no voltage-gated Na+ channels (3).

F8-20b

•K+ channels have opened in the initial segment (5) and the Na+ (6) ions cannot trigger an AP in that region since its absolutely refractory. Na+

ions initiate action potentials in segment (7).

F8-20c

Factors Influencing Conduction Speed of APs

• The resistance of the membrane to current leak out of the cell and the diameter of the axon determine the speed of AP conduction.

• Large diameter axons provide a low resistance to current flow within the axon and this in turn, speeds up conduction.

•Myelin sheath which wraps around vertebrate axons prevents current leak out of the cells. Acts like an insulator, for example, plastic coating surrounding electric wires.

• However, portions of the axons lack the myelin sheath and these are called Nodes of Ranvier. High concentration of Na+ channels are found at these nodes.

F8-6

Saltatory Conduction

• When depolarization reaches a node, Na+ enters the axon through open channels.

• At the nodes, Na+ entry reinforces the depolarization to keep the amplitude of the AP constant, but slows the current flow due to a loss of charge to the extracellular fluid.

• However, it speeds up again when the depolarization encounters the next node.

•The apparent leapfrogging of APs from node to node along the axon is called saltatoryconduction.

F8-22

Multiple Sclerosis

Jacqueline du Pré died of Multiple Sclerosis

• In demylinating diseases, such as multiple sclerosis, the loss of myelin in the nervous system slows down the conduction of APs. Multiple sclerosis patients complain of muscle weakness, fatigue, difficulty with walking and a loss of vision.

NEXT…

Membrane potential and its property…